This invention relates to improvements in electric band or strip heaters, and is more particularly concerned with such heaters which include novel organic bound ceramic strips sandwiching a resistance wire wound organic--ceramic heater core element, forming a unitary sub-assembly in such a heater structure.
In conventional band heaters of standard mica configuration, a wire wound mice heating element is assembled between mica insulator strips. The resultant mica sandwich is then encased in a sheet metal enclosure and formed into a desired shape. The electrical mica insulators used are of relatively low thermal conductivity and thus limit the heat transfer efficiency. Also these insulator strips undergo physical and chemical changes upon exposure to temperatures in excess of 1200.degree. F., which consist of dehydration or the baking out of the water of hydration. This change further decreases thermal conductivity and also reduces electrical insulating properties.
The presence of air voids and undesirable expansion under elevated temperature inherent in conventional mica heaters reduce heat transfer capability and result in loss of heater efficiency. These factors cause a conventional heater to operate at relatively higher than most efficient internal temperatures, resulting in premature heater failure. Additionally, where clamp force must be applied to maintain the heater in a given position, for example, around the nozzle of a tube having contents which must be heated as they pass therethrough, expansion of the heater under elevated temperatures causes loss of clamping force, resulting in heater inefficiency because the heater must be hotter to achieve a given surface temperature, and the higher temperature of the heater induces further expansion as the temperature is elevated.
In a second type of conventional band heater, coils of element wire are strung through ceramic insulator blocks which are shielded by a light sheet metal cover. Such an assembly is then strapped around an object to be heated. The resulting assembly can be likened to an oven assembly wherein heat transfer to the heated object is principally by convection rather than conduction. Such a heating system is not capable of high wattage because the inefficient convection heat transfer will not remove heat from the element wire fast enough, and thus would lead to over-temperaturing of the wire and premature element failure. This limitation of wattage thus increases heat up time of any object to be heated. Due to the open design of the casing for such conventional ceramic heaters, carbon forming materials can enter the heater, causing grounding type failures, which also may constitute a safety hazard. Also, inherent bulk requirements for such a heater, prevent the use of such conventional ceramic heaters in some applications where space is critical.
In the present invention, during fabrication of the heater, instead of a formed mica core and mica insulation strips, as in a conventional mica heater, and instead of a preformed wire strung ceramic block, as in a conventional ceramic heater, resistance wire is wound on a core strip of organically bound ceramic particles, which is sandwiched between similar organically bound ceramic strips, and the assembly is rolled or pressed in a metal housing to eliminate air voids between the elements, whereupon formation of the heater is completed and the entire assembly is heated to bake out the binders and sinter the ceramic particles into a unitary mass embedding therein the heater wire.
The novel organically bound ceramic particle strips each comprise a thin pliable `green` sheet of ceramic particles, pressed and rolled to a high density, and bonded together with binder materials, usually organic in nature, to an overall thickness upwards of 0.018 inch. The ceramic particles in the sheets are typical powdered ceramic materials, such as particles of aluminum oxide, magnesium oxide, boron nitride, or silicone dioxide. The binders for the ceramic particles are typically silicone, rubber, varnish, glyptal or the like. These bonded `green` or unbaked ceramic particle sheets conventionally are used in the fabrication of ceramic underlayment for printed circuits, the end product when baked out being referred to as "ceramic substrata", but in their `green` state before baking they are pliable and bendable.
In fabricating a heater according to the present invention, a lower organic--ceramic strip is laid over the bottom wall of a U-shaped metal housing, and the core organic--ceramic strip which has been wound with Nichrome or other resistance wire is placed over the lower strip. A second or upper organic--ceramic insulator strip is placed over the wire wound core strip, and a metal pressure plate is installed over the upper strip to close the housing. The edges of the housing are bent over the pressure plate, and the assembly is then rolled and flattened, thereby eliminating air voids between the elements and amalgamating and unifying the structure.
The assembly may then be shaped, for example bent into a curved band heater. When the heater assembly is in its final finished shape, the entire assembly is fired at an elevated temperature above the vaporization point of the binder materials in the strips and below the melting point of the sheath covering, preferably in an oxygen atmosphere, to vaporize and carbonize the binders and oxidize the carbon, which is vented from the heater in the form of carbon dioxide. As a result of this process, the ceramic materials of the strips agglomerate into an integral heat conducting and electrically insulating mass. Leads may then be connected to the heater element terminals and any desired heater mounting members may then be attached.